A leak resistant seal around an opening through a bulkhead is sometimes provided by using a bulkhead fitting. As illustrated in the cross-sectional diagram of FIG. 1, the fitting 1 typically has a cylindrically shaped flange 2 and an integral threaded member 3 that is normal to the flange with the axis of the threaded part coextensive with the axis of the flange. One side of the flange is designed to function as a sealing surface, which abuts against one side of the bulkhead, and an o-ring groove 4, containing a resilient o-ring 5, is formed within the sealing surface. The fitting is used to seal the opening through the bulkhead by passing the threaded member through the opening and fastening the fitting to the bulkhead by using a retaining nut 6 threaded onto the threaded member. When the retaining nut is fully screwed against the bulkhead, the resilient o-ring partially compresses against the other side of the bulkhead, thereby forming the seal.
Although several methods might be utilized to manufacture bulkhead fittings, one of the most desirable methods would be to manufacture the fittings using injection molding techniques that are capable of molding large volumes of fittings at relatively low cost. Unfortunately, however, attempting to mold a bulkhead fitting with the configuration described above presents some serious limitations. As illustrated in FIG. 2 the most economical manner to mold the fitting would be to utilize a pair of movable slide blocks 7 and 8, with the blocks configured to abut against a core plate 9. When the blocks and plate are assembled in this fashion they form a mold assembly having an internal cavity with an inside surface having the same shape as the outside surface of the bulkhead fitting. The bulkhead fitting would then be formed by injecting a liquid material, for example molten plastic, into the cavity and allowing the material to cool, thereby forming the fitting. The next step would be to separate the pair of slide blocks by moving them away from the threaded member and parallel to the cylindrical flange. But, separating the slide blocks in this fashion would not be possible without damaging the fitting due to the existence of that portion of the mold forming the o-ring groove, which effectively traps the fitting in the mold. Even if the fitting were somehow removed from the mold, for example while the fitting were still somewhat pliable, the fitting would retain the image of the pair of slide blocks in the form of a “parting-line”: a slightly raised portion of the fitting formed in a plane coextensive with the axis of the fitting and running through the fitting's sealing surface and o-ring groove. The portion of the parting-line extending through the sealing surface and o-ring groove would potentially compromise the ability of the o-ring to form a water-tight seal.
One technique to remove the fitting and avoid the formation of a parting-line would be to form a mold by utilizing a single block and a core plate as shown in FIG. 3. The fitting could then be removed by simply unscrewing the fitting from the block portion of the mold. This technique has the obvious draw back of consuming a lot of time and energy extracting the part from the mold, thereby negating the primary advantage of reducing the cost to mold the fitting. A further disadvantage is that there is an increased capital cost to manufacture the injection mold needed to make the fitting.
Another significant limitation in the technique of unscrewing the fitting from the mold is that the technique obviously only works if the fitting has a threaded member and the fitting is symmetrical about its longitudinal axis, otherwise the fitting could not be unscrewed from the mold. As shown in FIG. 4, this condition would exist if the fitting were adapted to be used as a fitting for a spa or hot tub, and the fitting contained one inlet for water 10 and another inlet for air 11, both of which protrude away from the fitting. This figure also illustrates the existence of a parting line 12 which is the result of using traditional methods of injection molding to form the apart.
Accordingly, what is needed is an improved method of molding a bulkhead fitting that allows the fitting to be quickly and easily removed from a mold assembly, without damaging the fitting and without compromising the ability of the fitting to form a water tight seal by creating a parting-line extending through the sealing surface and the o-ring groove.